U.S. Pat. No. 2,398,701 F. A. Firestone pertains to circuits for ultrasonic non-destructive testing wherein piezoelectric crystals are employed for both transmitting and receiving ultrasonic energy. Page 2 of that patent in the second column, lines 26-42, explains the necessity for a damping means on the back of the crystal in order to prevent "ringing" after the crystal is energized by a short pulse of ultrasonic frequency. Firestone suggests the use of a material such as Bakelite or lead for absorbing the ultrasonic energy. In U.S. Pat. No. 2,707,755 of Hardie et al., there is disclosed as a damping material a plastic matrix containing particles of metal (such as aluminum) or bubble inclusions. The plastic or particle density is graded so as to be greatest near the crystal for maximum energy transfer from the crystal into the damping material and becoming less dense with increasing distance so as to absorb the energy. Still later U.S. Pat. No. 2,972,068 of Howry et al. proposes as a backing material a synthetic resin containing a high concentration of a fine powder of heavy metal.
All of the proposed prior art backing materials have shortcomings which it is desirable to overcome. These include difficulty of fabrication, poor thermal, chemical, and mechanical stability, tendencies to shrink and creep, and problems of reproducibility. Furthermore, they are not usually electrically conducting.